1. Field of the Invention
The present invention is related to frequency synthesizers and, more particularly to closed loop frequency synthesizers with a wide operating frequency range.
2. Background Description
Frequency synthesizers, especially phase locked loop (PLL) synthesizers, may be required to operate over a wide frequency range. A typical PLL frequency synthesizer is a closed loop synthesizer that includes a voltage controlled oscillator (VCO) receiving a filtered steering line voltage that is generated by a charge pump, providing charges in response to frequency phase differences, i.e., between a reference frequency and the VCO output or some signal derived therefrom. The charge pump charges/discharges a capacitive load by passing a pumping charge (Q) to or from the load capacitor (C) to maintain a selected charge thereon as indicated by the desired quiescent voltage (V) across C. Since Q=CV, voltage across the load capacitor (C), is directly proportional to the charge on C. Thus, an active charge pump has three states: pumping charge onto the load; pumping charge off of the load; and neither, i.e., off. In its off state, the output of the charge pump must act as a high impedance (HiZ) similar to any well known three state driver.
Multi-band VCOs can be employed in frequency synthesizers to reduce the steering line voltage range required for the charge pump, but do not completely eliminate the above problems. Normally, the steering line voltage is again provided by a charge pump and filtered to control the frequency of the VCO. A simple example of a charge pump is a current source selectively connected to a load capacitance by a pair of independently controlled switches. The switches supply charge to or, remove charge from the capacitive load, by switching either or both of the switches on or off. Typically, the switches are transistors that do not switch on or off instantaneously, but involve some switching period during each reference cycle when both transistors are on. Any charge injection mismatch from the two switches, especially during switching, results in charge leakage to/from the load. This leakage causes an unintentional phase shift between the reference and loop frequencies. To compensate for this phase shift, the charge pump is turned on for a finite time during each cycle, which results in a voltage spur at the reference frequency and increases synthesizer inband noise because more charge pump noise is present.
Typical state of the art charge pumps can introduce significant switching noise and switching spur content into the frequency synthesizers. Reducing the operating range of the steering line voltage significantly reduces the noise but also constrains the design of the charge pump as well as reducing synthesizer operating range.
Thus, there is a need for a mechanism which reduces or eliminates charge pump related spur content and switching noise effects.